High frequency cavity press

ABSTRACT

An apparatus for applying pressure and a high frequency electric field to material comprising a cavity for containing the electric field which encloses platens relatively movable to apply pressure to material placed between the platens. The cavity electrically communicates with the platens for supplying the contained electric field to the platens which, in turn, cooperate with each other to apply the field to the material between the platens. The cavity and platens are also directly coupled into the means for generating the electric field as a resonant circuit.

United States Patent 1191 Bradley June 26, 1973 HIGH FREQUENCY CAVITYPRESS [75 Inventor: Robert W. Bradley, Marblehead, m Goldberg MassAssistant Examiner-Hugh D. Jaeger Att0rney-Richard A. Wise, Richard B.Megley and [73] Assignee: USM Corporation, Boston, Mass. Willi m R, E n

[22] Filed: May 26, 1971 21 Appl. No.: 147,083 [57] ABSTRACT Anapparatus for applying pressure and a high frequency electric field tomaterial comprising a cavity for [52] US. Cl. 219/10.81, 156/273containing the electric field which encloses platens rah hat. Cl.atively movable to pressure to material placed [58] Field of Search219/10.5 5, 10.61, between the platens The cavity electrically communi219/1063 1 156/273 cates with the platens for supplying the containedelectric field to the platens which, in turn, cooperate with [56]References C'ted each other to apply the field to the material betweenUN TED ST S PATENTS the platens. The cavity and platens are alsodirectly 3,205,114 9/1965 Gross 156/273 X coupled into the means forgenerating the electric field 3,469,054 9/1969 Serota 2l9/l0.6l as aresonant circuit. 3,401,248 9/1968 Kim 156/273 x 3,357,108 12/1967Bennett 219 1061 x 14 Clams, 3 Drawing Flgures ll 1 7 o [llllllllllllillllll HIGH FREQUENCY CAVITY PRESS BACKGROUND OF THE INVENTIONThis invention relates to an apparatus for applying pressure and a highfrequency electric field to material.

Machines for applying a high frequency electric field to material arewell known for their utility in heating dielectric material. Frequently,such high frequency heating machines are used on dielectric materialwhich is heat softenable. Usually, a die is applied to the softenedmaterial to emboss the surface of the material.

The high frequencies which have been found useful for heating materialare in the mega-Hertz range. These frequencies have correspondingwavelengths of a few meters. Such wavelengths are in the same order ofmagnitude as the physical dimensions of conveniently sized machines forworking on the material. It is known that electromagnetic waves whichhave wavelengths similar in size to the physical dimensions ofconductive parts of a machine on which they are impressed tend toradiate from the conductive parts.

Only the conductive parts of high frequency heating machines adjacentthe material are designed to pass electric currents through the materialto heat the material. However, some electric voltages impressed on thework do not pass through the material but instead radiate as waves intospace and perform no useful function. The radiated waves thus reduce theefficiency of the machine and often interfere with radiation frequenciesreserved for communications and other purposes requiring such radiation.Since radiation from a high frequency heating machine is an unwantedby-product, the frequencies and signal strengths at which such radiationis permitted are strictly limited by government regulations.

In many instances it has been found impossible to prevent inefficientand illegal radiation from conventional high frequency machines. Onesolution to this problem has required placing the machine in a specialelectrically shielded room. Not only are such rooms expensive toconstruct, but the location of the machine in the room may interferewith the logistics of production line operation. These problems haveseverely hampered the use of high frequency heating machines. Anothersolution involves operating high frequency machines at fixed,governmentally approved frequencies and signal strengths; this may beinconveniently limiting.

Solving still another problem requires impedance matching or tuning theelectric field generating and field applying components of a machine toeach other. It has long been known that such tuning reduces thereflection of electric energy from the interface of electricalcomponents toward the high frequency source. Here, reflected energy doesnot reach the'field applying means and thus reduces the efficiency ofthe machine. The size and the type of material between the platenseffects the impedance and thus the reflection of the machine. Eachchange of material thus requires tuning of the machine for efficientoperation of conventional machines.

SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provide a high frequency machine which does not radiateelectromagnetic waves into the surrounding space. It is a further objectof the invention to eliminate the need to tune the machine components toeach other for varying conditions of machine use. Still a further objectof the invention is to provide a machine in which the elapsed time toheat material from one temperature to another is independent of thesurface area of the material.

To this end a high frequency machine is provided with a cavity forcontaining an electric field and enclosing the parts of the machinewhich have high frequency energy on them. The cavity is electricallyconducting on its interior surface and designed so that substantially nohigh frequency energy appears on its exterior surface. The cavity has anopening through which pieces of material to be operated upon may beplaced between platens in the cavity. A cover is positioned relative tothe opening to contain the electric field at the opening. Platens aremovably mounted within the cavity and cooperate to press the materialplaced between them. The platens are also electrically connected to thecavity to apply the high frequency field contained in the cavity to thematerial between the platens. Means for supplying a high frequency fieldto the cavity are provided.

The means for supplying the electric field to the cavity directly couplea high frequency power supply to the machine which itself serves as aresonant tank circuit to form the high frequency generator. The materialin the machine forms an integral part of the resonant circuit directlycoupled into the high frequency generator. Thus changes in the materialmerely change the frequency of the machine. A range of frequencies isacceptable for treating the material and, since the cavity preventsradiation,' governmental frequency regulations are not a problem. Thedirect coupling of the machine or field applying component as part ofthe high frequency generator or field generating component eliminatesenergy reflection problems between field applying and generatingcomponents by combining them as one integral component. The cavity anddirect coupling thus eliminate the need to tune the machine.

It is additionally possible and preferable to select impedencecharacteristics for the machine resonant circuit relative to theinternal impedance characteristics of the high frequency power supplysuch that the electric power of the high frequency field applied to thematerial is directly proportional to the effective area of the platensfor a wide range of effective areas, while the potential differenceacross the platens or from one platen to the other is held constant. Theeffective area of each platen is that surfacearea of each platen whichaffects the material between the platens. The relative values of thecharacteristics which produce the direct proportionality are empiricallydetermined. The direct proportionality results in the application topieces of the material uniform except in area of constant electric fieldpower per unit area regardless of the area of mate'- rial between theplatens. In a high frequency heating machine the power per unit area isthe calories per second per unit area applied to the material; thus theelapsed time to heat the material from one temperature to any othertemperature is independent of the area of the material. The constantpotential difference across the platens may conveniently be set belowthat potential which will permit arcing across the platens, a valuedependent on ambient conditions at the press. In a high frequencyheating machine such arcing may damage the material being heated.

BRIEF DESCRIPTION OF THE DRAWINGS The above and other features of theinvention will now be more particularly described with reference to thedrawings of a preferred embodiment of the invention in which:

FIG. 1 is a perspective view, partly in section, of an apparatusembodying the invention;

FIG. 2 is a side view, partly in section, of the'apparatus shown in FIG.1; and

FIG. 3 is a side view, partly in section, of a support portion of theapparatus shown in FIG. 2.

DESCRIPTION OF THE PREFERRED EMBODIMENT As seen in FIG. 1, a highfrequency heating machine embodying the instant invention has a cavitygenerally indicated at enclosing an operating portion of the machine andelectrically conducting on its interior surfaces. The cavity has anopening indicated generally at 12 to provide access into the cavity forpieces of material 14 to be heated in the machine. In FIG. 2 a platen 16is seen to be mounted within the cavity on a support structure indicatedgenerally at 18. Another platen is mounted for movement relative to thefirst platen to press material 14 between the platens in the cavity. Acomposite cover 22 for the cavity opening 12 is secured to the movableplaten. When the platens engage and press the material, the cover closesthe opening 12 to contain an electric field in the cavity. The electricfield is applied to the cavity through input link 23 from a highfrequency power supply (not shown) directly coupled to the cavity. Theplaten l6 electrically communicates with the cavity and the secondplaten also electrically communicates with the cavity through thecomposite cover to apply the electric field in the cavity to thematerial 14 between the platens. The electric field applied to thematerial by the platens dielectrically heats lossy material. Preferably,a die 25 is placed between the platens to emboss the heated materialwhen it is pressed by the platens.

The cavity 10 is a box-like structure having interior walls 40 and abottom 42 made from an electrically conducting material such as copper.At the top of the box is the opening 12 through which material to beheated is inserted into the cavity. Additional walls 44 project from acentral portion of the cavity bottom 42 to enclose a central supportshaft 46 of the platen support structure 18. The walls 44 shield theshaft 46 from the electric field in the cavity. Supporting walls 48physically and electrically connect the upper end of walls 44 to theperiphery of the first platen 16. The physical connection of the walls48 to the platen 16 supports the entire cavity 10 which is notsupportingly connected to any other part of the machine to isolate thecavity and contain the electric field in the cavity. The containedelectric field does not substantially radiate into the space adjacentthe machine.

The support structure 18 supports the first platen l6 and thus alsosupports the cavity 10 connected to the platen. The support structure 18comprises the central support shaft 46 and additional support members 50which project through close fitting holes 51 (FIG. 3) in the cavity base42 and supporting walls 48 to support the first platen 16 near itsperiphery. Although it is intended, in this preferred embodiment of theinvention, that the central support 46 provide the main physical loadbearing support while the members prevent deflection of the platen 16near its periphery, supports of either the types illustrated by 46 and50 may be used separately or in any other desired combination.

The support shaft 46 may be made from any material selected for itssupporting strength because the enclosing walls 44 and 48 of the cavityprevent any of the electric field in the cavity from reaching thesupport. Thus, the central support will not conduct any part of theelectric field from the cavity to the space surrounding the machine. Thesupport members 50 must be made from an electrically non-conductingmaterial such as ceramic to prevent them from conducting the electricfield from within the cavity to the space outside the cavity. The holes51 in the cavity walls 42 and 48 through which the support members 50project should be sufficiently close fitting to cooperate with themembers 50 to contain the electric field in the cavity.

The platen 116 is mounted on the support structure 18 to provide a rigidplatform against which the movable platen 20 can press the material 14.The platen 16 may be made from a strong, rigid material such as steel.Preferably, the platen 16 supports on its upper surface a panel 60, ofsilicone glass or the like, which extends beyond the edges of the platento the walls 40 of the cavity. The panel thus serves as an insulatorplate and as a cover for the opening in the cavity to exclude dust andforeign matter. The platen i6 and panel 60 preferably support a secondpanel 62 which is substantially coextensive with the area of the platen.The second panel 62 may be made of silicone rubber or the like andprovides a resilient surface on which the material 14 may be placed. Thefirst and second panels also affect the electrical capacitance betweenthe platens 16 and 20.

The platen 20 is mounted for vertical movement relative to the platen 16and is moved by a fluid operated cylinder 63 to press the work piece 14between the platens. Electrically operated thermal-heating elements 64are embedded interior the platen 20 and are directly connected to asource of electric power (not shown) to thermally heat the secondplaten. The elements 64 are embedded in the platen 20 sufficiently farfrom the inner surface 65 of the platen 20 so that no high frequencyenergy reaches the elements due to the known skin effect phenomena inwhich high frequency energy appears only on the surface of conductors.Thus no filters or other devices are needed to protect electricallyconnected external components, here the electric power source for theheating elements 64, from the high frequency energy in the cavity.

A die 25 is secured to the movable platen 20 to engage the material 14when the platen 20 is moved toward the fixed platen 16. The materialengaging surface 66 of the die may be embossed with any characteristicswhich it is desired to impress upon the material 14. Because the diesurface 66 more closely approaches the fixed platen 16 than does themovable platen 20, the area of the die surface 66 alters the capacitancebetween the platens by altering the capacitively effective area of theparallel plate platen structure creating the capacitance. The die areathus forms the effective platen area. A flange 69 electrically connectsthe die to the cover 22.

The composite cover 22 is secured at an inner edge 70 to the platen 20and flange 69 and is secured at its outer edge by a bracket 68 to anouter cover 71. The

cover 22 is made from electrically conducting material and has a jog 72intermediate its edges to permit the cover to bend to accommodatethermal expansion of the platen 16 when it is thermally heated by theheating elements 64. The support bracket 68 also electrically connectsthe cover 22 to contact members 74 which extend around the entireperiphery of the cover. When the cover is closed and the die 25 engagesthe material 14 on the panel 62, the contact members electricallyconnect the cover with cooperative contact members 76 electricallyconnected to the walls 40 of the cavity. Through cooperative contacts 74and 76 the cover electrically connects the walls 40 of the cavity to themovable platen 20 and die 25 to close the cavity opening 12 and containthe electric field within the cavity.

A high frequency electric energy power supply (not shown) is directlycoupled to the top surface of the first platen by a lug (not shown) ofinput terminal 23. The input terminal includes capacitor 79 which blocksdirect current input to the cavity. Most of the high frequency energyfrom the input radiates as a rapidly changing electric field between thesurface of the fixed platen and the die and movable platen. The die andmovable platen electrically communicate with the cavity walls, theinterior surfaces of which electrically communicate with a groundingplate 80. Only the interior surface of the electrically communicatingelements need be considered since by a well known phenomena, commonlycalled skin effect, all the high frequency energy on the cavity remainson the surface of the cavity. Some of the high frequency energycirculates in the space enclosed by the cavity. Part of this energy isreceived by a pick up coil 82 to provide feedback for the high frequencypower supply. Preferably the coil 82 is of an oriented type, such as theloop coil shown, so that the feedback may be varied by rotating the coilto affect generator performance in a well known manner.

The interior conducting surface of the cavity is an inductance at thehigh frequencies employed in the machine. The parallel plate structureof the platens, panels, material and die is capacitive and together withthe associated capacitor 79 are in parallel with the cavity. The machinethus forms an inductive-capacitive parallel resonant tank circuitdirectly coupled into the high frequency generator. Direct coupling isnot by transmission line, but by direct link to the power supply portionof the generator.

It has been found experimentally that direct proportionality between theeffective area of the platens and the power in the machine forapplication to the material is achieved for a high frequency powersupply cornprising a plate loaded vacuum tube having DC characteristicsin which the plate current is 2 amps; grid current, one-half amp; platevoltage, 3,400 volts and gain, mu 30; and a cavity 10 having a bottom42-l7 X l7 inches, walls 40-l0 inches high, walls 44-9 inches high andwalls 48-4 k inches wide, panels 60 and 62% inches thick each and anassociated capacitor 79 of 0.0005 microfarads.

It is understood that the preferred embodiment described above is merelyillustrative of the invention and not a limitation to the invention.

Having thus described my invention, what I claim as new and desire tosecure by Letters Patent of the United States is:

l. A machine for applying a high frequency electric field and pressureto material which comprises:

means defining a cavity for containing the field in the cavity having anopening for admitting the material into the cavity;

a cover positioned relative to the opening for containing the field inthe cavity at the opening;

' platens within the cavity for receiving the material between them andfor applying the field to the material;

means electrically connecting one of the platens and the cavity definingmeans for applying high frequency energy to the one platen;

means for supplying the high frequency energy to the other platen andcavity; and

means for supporting the platens for relative movement with respect toeach other for applying the pressure to the material.

2. A machine as in claim 1 wherein the cover is physically connected toone of the platens for movement therewith.

3. A machine as in claim 1 wherein the platen support means comprises ashaft projecting through a hole in the cavity defining means to one ofthe platens and enclosed by additional walls of the cavity extendingfrom the platen on the shaft to the hole.

4. A machine as in claim 3 wherein the additional walls are physicallyconnected to the shaft supported platen and the cavity defining meansfor supporting the cavity defining means.

5. A machine as in claim 3 wherein the additional walls comprise theelectrically connecting means.

6. A machine as in claim 1 wherein the platen support means comprises anelectrically non-conducting member projecting through a hole in thecavity defining means to the platen on the member.

7. A machine as in claim 1 wherein the platens and cavity defining meanselectrically cooperate to form a resonant circuit.

8. A machine as in claim 7 wherein the means for applying the fieldcomprises the resonant circuit.

9. A machine as in claim 7 wherein the resonant circuit is directlycoupled to a high frequency power supply to form a high frequencygenerator.

10. A machine as in claim 1 wherein the power of the electric field inthe machine is directly proportional to an effective surface area ofeach platen to apply a substantially constant field power'per unit areato pieces of material of various areas.

11. A machine as in claim 10 wherein a potential difference across theplatens is constant.

12. A machine for applying a high frequency electric field to material,comprising:

a generator for high frequency electric energy; and

means directly coupled into the generator for applying the energy to thematerial as a field having a substantially constant power per unit areaof the material for pieces of the material of various areas.

13. A machine as in claim 12 wherein the directly coupled meansadditionally comprise a resonant circuit portion of the high frequencygenerator.

14. A machine for applying a high frequency electric field and pressureto material, comprising:

a generator for high frequency electric energy; two platens electricallyconnected to the generator for applying the energy as an electric fieldand movable for applying the pressure to the material between theplatens; and means electrically connected to the generator and platensfor providing the power of the field in direct proportion to aneffective area of the platens.

1. A machine for applying a high frequency electric field and pressureto material which comprises: means defining a cavity for containing thefield in the cavity having an opening for admitting the material intothe cavity; a cover positioned relative to the opening for containingthe field in the cavity at the opening; platens within the cavity forreceiving the material between them and for applying the field to thematerial; means electrically connecting one of the platens and thecavity defining means for applying high frequency energy to the oneplaten; means for supplying the high frequency energy to the otherplaten and cavity; and means for supporting the platens for relativemovement with respect to each other for applying the pressure to thematerial.
 2. A machine as in claim 1 wherein the cover is physicallyconnected to one of the platens for movement therewith.
 3. A machine asin claim 1 wherein the platen support means comprises a shaft projectingthrough a hole in the cavity defining means to one of the platens andenclosed by additional walls of the cavity extending from the platen onthe shaft to the hole.
 4. A machine as in claim 3 wherein the additionalwalls are physically connected to the shaft supported platen and thecavity defining means for supporting the cavity defining means.
 5. Amachine as in claim 3 wherein the additional walls comprise theelectrically connecting means.
 6. A machine as in claim 1 wherein theplaten support means comprises an electrically non-conducting memberprojecting through a hole in the cavity defining means to the platen onthe member.
 7. A machine as in claim 1 wherein the platens and cavitydefining means electrically cooperate to form a resonant circuit.
 8. Amachine as in claim 7 wherein the means for applying the field comprisesthe resonant circuit.
 9. A machine as in claim 7 wherein the resonantcircuit is directly coupled to a high frequency power supply to form ahigh frequency generator.
 10. A machine as in claim 1 wherein the powerof the electric field in the machine is directly proportional to aneffective surface area of each platen to apply a substantially constantfield power per unit area to pieces of material of various areas.
 11. Amachine as in claim 10 wherein a potential difference across the platensis constant.
 12. A machine for applying a high frequency electric fieldto material, comprising: a generator for high frequency electric energy;and means directly coupled into the generator for applying the energy tothe material as a field having a substantially constant power per unitarea of the material for pieces of the material of various areas.
 13. Amachine as in claim 12 wherein the directly coupled means additionallycomprise a resonant circuit portion of the high frequency generator. 14.A machine for applying a high frequency electric field and pressure tomaterial, comprising: a generator for high frequency electric energy;two platens electrically connected to the generator for applying theenergy as an electric field and movable for applying the pressure to thematerial between the platens; and means electrically connected to thegenerator and platens for providing the power of the field in directproportion to an effective area of the platens.